Charger circuit and PWM controller thereof

ABSTRACT

A PWM controller includes a short circuit mode circuit which has input ends connecting simultaneously to a power supply input end and an output end of an output driving circuit, and an oscillator which has a temperature compensation circuit that includes a medium value multi-transistor resistor of a negative temperature coefficient and a well resistor of a positive temperature coefficient. The invention also provides a charger circuit which includes the PWM controller set forth above and a constant voltage/current control circuit coupling with the PWM controller through a transformer. A system adopted the PWM controller circuit and charger circuit of the invention does not need a voltage stabilization diode now equipped in many conventional designs. The invention provides an improved short circuit protection, eliminates low frequency harmonic waves, packages power transistors and the controller into a single TO-94, increases the cluster density and enhances the reliability of small packages.

FIELD OF THE INVENTION

The present invention relates to pulse width modulation (PWM) andparticularly to a lower power PWM control technique related to chargercircuits and PWM controllers thereof.

BACKGROUND OF THE INVENTION

At present the chargers for handsets and digital cameras mostly adopt aswitch power supply circuit. They are designed for a lower power supplyand can be classified as follow: a first type in which the circuit onthe primary side of a transformer adopts RCC separated elements to formself actuation control or a PWM controller to form external actuationcontrol; a second type in which the secondary side of a transformeradopts a dual operational charge circuit to achieve constant current anda voltage base circuit to achieve constant voltage, or adopts a voltagebase circuit to achieve constant voltage and a transistor to achieveconstant current; and a third type which adopts an ASIC (applicationspecific integrated circuit) design which has a dedicated integratedcircuit on the primary side of a transformer to control output atconstant voltage and constant current without an optical coupler and aconstant voltage/current controller on the secondary side of thetransformer.

The first type is simpler, but has notable disadvantages, such asgreater element dispersion, lower efficiency, no short circuit function,lower production yield, thus the acceptance is dwindling now. The thirdtype that adopts the ASIC design is simpler, but it requires a specialtechnology available only to a small number of manufacturers. Moreover,it does not provide a vigorous voltage/current control circuit, hencesystem harmonic wave and noise performances are less desirable, and thecost also is higher, thus it still cannot fully meet customers'requirements. The second type is most widely adopted. Its systemperformance can meet the constant voltage/current requirement of mosthigh performance chargers. Environmental temperature variation also doesnot have significant impact on the system performance. However, it alsohas its share of problems, such as system cost is higher and circuitboard area is greater.

To overcome the aforesaid problems, a higher performance solution isneeded to provide a higher efficiency, improved short circuitcharacteristics, and lower system output harmonic wave and system cost.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a highperformance charger circuit that has a higher efficiency, improved shortcircuit characteristics, lower system output harmonic wave and systemcost.

The invention also provides a PWM controller which has a short circuitmode circuit in which input ends are simultaneously connected to a powersupply input end and an output end of an output driving circuit. The PWMcontroller also has an oscillator which includes a temperaturecompensation circuit adopting a medium value multi-transistor resistorof a negative temperature coefficient and a well resistor of a positivetemperature coefficient.

The foregoing, as well as additional objects, features and advantages ofthe invention will be more readily apparent from the following detaileddescription, which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a conventional charger circuit in a PWMcontroller.

FIG. 2 is a chart showing a charging curve of a conventional chargercircuit.

FIG. 3 is a circuit block diagram of an embodiment of the PWM controllerof the invention.

FIG. 4 is a fragmentary circuit block diagram of an embodiment of ashort circuit mode circuit in the PWM controller of the invention.

FIG. 5 is a fragmentary circuit diagram of an embodiment of a frequencytemperature compensation circuit in an oscillator of the invention.

FIG. 6 is a circuit diagram of another embodiment of the charge circuitof the invention.

FIG. 7 is a chart showing a charging curve of another embodiment of thecharge circuit of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIG. 3 for a circuit block diagram of an embodiment of aPWM controller 100 of the invention. It includes:

a power on circuit 101 which is connected to a power supply input end001 and determines the interim threshold duty voltage of the powersupply input end 001 at the power on stage and the minimum duty voltageduring normal operation;

an oscillator 108 which generates a square wave signal at a constantfrequency and has an output end connecting to a PWM logic circuit 107and a positive and negative temperature compensation circuit to generatethe constant frequency used by the power supply;

a current limit comparator 106 which has an input end connecting to thepower supply input end 001 in response to a current sampling signal ofan output driving circuit 104 to carry out feedback of a currentcircuit. The current limit comparator 106 also responds to voltagevariations of the power supply input end 001 to carry out feedback of avoltage circuit. Current feedback signals and voltage feedback signalsare sent to the PWM logic circuit 107 in an error signal format throughthe current limit comparator 106;

the PWM logic circuit 107 which is connected to the oscillator 108 torespond to the square wave signal output therefrom and also is connectedto the current limit comparator 106 to receive the error signal thereofto determine the duty cycle of output driving pulse. The PWM logiccircuit 107 further responds to input signals of a short circuit modecircuit 103 and periodically stops output signals to protect the system;

the short circuit mode circuit 103 which has one input end connecting tothe power supply input end 001 and another input end connecting to anoutput end of the output driving circuit 104. During normal operationthe voltage of the output end of the output driving circuit 104 ishigher and the voltage of the power supply input end 001 is lower. Inthe event of short circuit or a light loading condition and the outputend voltage of the output driving circuit 104 is lower, the voltage ofthe power supply input end 001 is higher, the short circuit mode circuit103 makes the PWM controller 100 to enter a short circuit protectionmode; and

the output driving circuit 104 which has an input end connecting to thePWM logic circuit 107 and output ends connecting to the short circuitmode circuit 103 and the current limit comparator 106 to output the PWMpulse signals. It is connected to and drives a power transistor 105outside the PWM pulse controller 100 through power elements locatedinside the PWM controller 100.

The power on circuit 101 may also be connected to an internal basecircuit 102 to provide internal base signals.

As may be seen from the embodiment shown in FIG. 3, the PWM controllerof the invention has features as follow: the input ends of the shortcircuit mode circuit 103 are connected simultaneously to the powersupply input end 001 and the output end of the output driving circuit104. Refer to FIG. 4 for a fragmentary circuit block diagram of anembodiment of the short circuit mode circuit 103. The short circuit modecircuit 103 of the PWM controller 100 of the invention adopts a controlmethod different from the conventional PWM controller. The conventionalPWM controller receives input signals merely from the output drivingend, equivalent to the output driving circuit 104 of the invention, butdoes not receive the input signals at the same time from the powersupply input end 001, hence response speed is slower and not accurate.By contrast, the short circuit mode circuit 103 of the invention isconnected to the power supply input end 001 and the output end of theoutput driving circuit 104 at the same time, thus in normal operationcondition the voltage at the output end of the output driving circuit104 is higher, and the voltage at the power supply input end 001 islower. In the event of short circuit or a light loading condition thevoltage at the output end of the output driving circuit 104 is lower,and the voltage at the power supply input end 001 is higher. By means ofsuch a design the system can enter the short circuit protection modewhen short circuit occurs as previously discussed. It is to be notedthat the PWM controller 100 does not enter the short circuit protectionmode in the light loading condition.

The temperature compensation circuit in the oscillator 108 of the PWMcontroller 100 of the invention adopts a medium value multi-transistorresistor of a negative temperature coefficient and a well resistor of apositive temperature coefficient. Refer to FIG. 5 for a fragmentarycircuit diagram of the frequency temperature compensation circuit in theoscillator of an embodiment of the invention. The frequency of theoscillation circuit usually is determined by a capacitor C0 and powersupplies I0 and I1. Current I flows into one power supply I0, andcurrent 4I flows into another power supply I1. They are obtained throughtransistors M0 and MI and a mirror image bias current Ibias of othercurrent mirror. In the general CMOS design the bias current Ibias isgenerated by applying a positive temperature coefficient Δ Vbe (deltaVbe shown in the drawings) to resistors R1 and R2. As there is notemperature coefficient of a resistor that can exactly match thetemperature coefficient of Δ Vbe, the temperature characteristics of thecurrent is not desirable, and frequency stability is affected. The get adesired positive temperature coefficient of Δ Vbe, the oscillationcircuit of the invention employs a medium value multi-transistor R1 of anegative temperature coefficient and a well resistor R2 of a positivecoefficient to reduce the temperature coefficient of the bias current.The formula is as follow:

Ibias=Δ Vbe/(R2+R1).

The temperature coefficient is determined by the following formula:

$\frac{\partial{Ibias}}{\partial T} = {{\frac{1}{{R\; 1} + {R\; 2}}\frac{{\partial\Delta}\; {Vbe}}{\partial T}} + {\frac{{- \Delta}\; {Vbe}}{\left( {{R\; 1} + {R\; 2}} \right)^{2}}\frac{{\partial R}\; 2}{\partial T}} + {\frac{{- \Delta}\; {Vbe}}{\left( {{R\; 1} + {R\; 2}} \right)^{2}}\frac{{\partial R}\; 1}{\partial T}}}$

Based on the above two formulas, it can be seen that as the temperaturecoefficient of R2 is greater, the sum of two previous items is negative.This results in a not desirable current temperature coefficient. Byincluding the resistor R1 of a negative temperature coefficient, thetotal is approaching zero. Hence a desired current temperaturecoefficient that can meet requirements can be obtained. And theoscillation frequency of a greater temperature stability can beobtained.

The power elements inside the PWM controller 100 set forth above may bebuilt-in power transistors. And the power transistor 105 and PWMcontroller 100 can be packaged in a single TO-94 to save space.Moreover, the power on circuit 101 in the PWM controller 100 of theinvention does not include a voltage stabilization diode.

By means of the design previously discussed, the invention provides animproved PWM controller 100. A system adopted the invention does notneed a voltage stabilization diode now exists in many of theconventional techniques. It also provides improved short circuitprotection characteristics than the conventional designs.

Refer to FIG. 6 for the circuit diagram of another embodiment of thecharger circuit of the invention. The charger circuit includes twoportions, namely a PWM controller portion and a constant voltage/currentcontrol circuit portion that are coupled through a transformer T1. ThePWM controller 401 shown in FIG. 6 is same as the PWM controller 100 inFIG. 3. It has pins sequence numbers 1-4 corresponding to input/outputend numbers 001-004 in FIG. 3. It has a constant voltage/current controlcircuit marked by 402 in FIG. 6. The rest elements are auxiliaryelements. The operating principle of the charger circuit 400 is asfollow: The voltage after rectification is sent to the PWM controller401 through the transformer T1 to provide power on energy at the powersupply input end of the PWM controller 401 to complete system power onprocess. In normal duty conditions, the auxiliary winding of thetransformer T1 provides power supply energy for the PWM controller 401.A diode D8 is connected to a resistor R12 in series to provide requiredpower for the constant voltage/current control circuit 402. Thusconstant current characteristics can be maintained when output voltageis in the range of 2.5V-5V, and short circuit power is less than 1 watt.The harmonic wave also is lower in the no loading condition. Refer toFIG. 7 for the charging curve of the charger circuit 400. In thisembodiment the constant voltage/current control circuit 402 can alsoprovide short circuit control. Hence the charger circuit of theinvention can get a very low output voltage harmonic wave in the noloading condition.

Referring to FIG. 1 for the circuit diagram of a charger circuit of aconventional PWM controller. It also has a PWM controller coupling witha constant voltage/current circuit through a transformer. However itdiffers from the invention as follow: The structure of the PWMcontroller is different and coupling of the auxiliary circuits also isdifferent, and implementation of the constant voltage/current circuitalso is different. The invention employs a core plate, while theconventional technique adopts a dual operational charge circuit. Theembodiment shown in FIG. 1 is a conventional three-end PWM controllerincluding a standard processing amplifier to achieve constant voltageand current.

FIG. 2 depicts the charge curve of the charger circuit shown in FIG. 1.The charged circuit shown in FIG. 6 has disadvantages, namely theconstant current range is narrower (3V-5V), and the charging curve isalso less desirable, and short circuit current is greater (2 A).Moreover, the system needs a great number of separated elements and thecost is higher.

In short, a system adopted the PWM control circuit and charger circuit400 of the invention does not need a voltage stabilization diode thathas to be used in many of the conventional designs. Short circuitprotection characteristics are better than many of the existing designs.It adopts a novel approach to eliminate the low frequency harmonic wave.The power transistor and the controller can be packaged in a singleTO-94, hence cluster density is enhanced. It overcomes the reliabilityproblem occurred to the small package. The invention provides a highperformance design at a lower cost that has a higher efficiency,improved short circuit characteristics and a lower system outputharmonic wave.

While the preferred embodiments of the invention have been set forth forthe purpose of disclosure, modifications of the disclosed embodiments ofthe invention as well as other embodiments thereof may occur to thoseskilled in the art. Accordingly, the appended claims are intended tocover all embodiments which do not depart from the spirit and scope ofthe invention.

1. A PWM controller, comprising the characteristic that a short circuitmode circuit has input ends connecting respectively and simultaneouslyto a power supply input end and an output end of an output drivingcircuit, and an oscillator which has a temperature compensation circuitwhich includes a medium value multi-transistor resistor of a negativetemperature coefficient and a well resistor of a positive temperaturecoefficient.
 2. The PWM controller of claim 1 further comprising: apower on circuit which is connected to the power supply input end anddetermines an interim threshold duty voltage of the power supply inputend at the power on stage and a minimum duty voltage during normaloperation; the oscillator which generates a square wave signal at aconstant frequency and has an output end connecting to a PWM logiccircuit and a positive and negative temperature compensation circuit togenerate the constant frequency used by a power supply; a current limitcomparator which has an input end connecting to the power supply inputend in response to a current sampling signal of the output drivingcircuit to carry out feedback of a current circuit, and responds tovoltage variations of the power supply input end to carry out feedbackof a voltage circuit, current feedback signals and voltage feedbacksignals being sent to the PWM logic circuit in an error signal formatthrough the current limit comparator; the PWM logic circuit which isconnected to the oscillator to respond to the square wave signal outputtherefrom and is connected to the current limit comparator to receiveerror signals thereof to determine duty cycle of output driving pulses,and responds to input signals of the short circuit mode circuit andperiodically stops output signals to protect the system; the shortcircuit mode circuit which has one input end connecting to the powersupply input end and another input end connecting to the output end ofthe output driving circuit; during the normal operation the voltage atthe output end of the output driving circuit is higher and the voltageat the power supply input end is lower; in the event of short circuit ora light loading condition and the output end voltage of the outputdriving circuit is lower and the voltage at the power supply input endis higher; the short circuit mode circuit making the PWM controller toenter a short circuit protection mode; and the output driving circuitwhich has an input end connecting to the PWM logic circuit and outputends connecting to the short circuit mode circuit and the current limitcomparator, and generates PWM pulse signals and is connected to anddrives a power transistor outside the PWM controller through powerelements located in the PWM controller.
 3. The PWM controller of claim2, wherein the power elements in the PWM controller are built-in powertransistors.
 4. The PWM controller of claim 3, wherein the powertransistors and the PWM controller are package in a TO-94.
 5. The PWMcontroller of claim 2, wherein the power on circuit does not include avoltage stabilization diode.
 6. A charger circuit comprising a PWMcontroller and a constant voltage and constant current control circuitcoupled with the PWM controller through a transformer, wherein: the PWMcontroller has a short circuit mode circuit which has input endsconnecting respectively and simultaneously to a power supply input endand an output end of an output driving circuit, and an oscillator whichhas a temperature compensation circuit which includes a medium valuemulti-transistor resistor of a negative temperature coefficient and awell resistor of a positive temperature coefficient.
 7. The chargercircuit of claim 6, wherein the PWM controller includes: a power oncircuit which is connected to the power supply input end and determinesan interim threshold duty voltage of the power supply input end at thepower on stage and a minimum duty voltage during normal operation; theoscillator which generates a square wave signal at a constant frequencyand has an output end connecting to a PWM logic circuit and a positiveand negative temperature compensation circuit to generate the constantfrequency used by a power supply; a current limit comparator which hasan input end connecting to the power supply input end in response to acurrent sampling signal of the output driving circuit to carry outfeedback of a current circuit, and responds to voltage variations of thepower supply input end to carry out feedback of a voltage circuit,current feedback signals and voltage feedback signals being sent to thePWM logic circuit in an error signal format through the current limitcomparator; the PWM logic circuit which is connected to the oscillatorto respond to the square wave signal output therefrom and is connectedto the current limit comparator to receive error signals thereof todetermine duty cycle of output driving pulses, and responds to inputsignals of the short circuit mode circuit and periodically stops outputsignals to protect the system; the short circuit mode circuit which hasone input end connecting to the power supply input end and another inputend connecting to the output end of the output driving circuit; duringthe normal operation the voltage at the output end of the output drivingcircuit is higher and the voltage at the power supply input end islower; in the event of short circuit or a light loading condition andthe output end voltage of the output driving circuit is lower and thevoltage at the power supply input end is higher; the short circuit modecircuit making the PWM controller to enter a short circuit protectionmode; and the output driving circuit which has an input end connectingto the PWM logic circuit and output ends connecting to the short circuitmode circuit and the current limit comparator, and generates PWM pulsesignals and is connected to and drives a power transistor outside thePWM controller through power elements located in the PWM controller. 8.The charger circuit of claim 7, wherein the power elements in the PWMcontroller are built-in power transistors.
 9. The charger circuit ofclaim 8, wherein the power on circuit does not include a voltagestabilization diode.
 10. The charger circuit of claim 9, wherein theconstant voltage and constant current control circuit provides shortcircuit control.